Unless you're a ripped bodybuilder, at 210 lbs you still have a fair amount of fat under your skin. Almost everyone I've ever seen with loose skin still has some fat under it making it sag. It still has some tension on it so it has no reason to shrink. So, take a deep breath, congratulate youself on accomplishing what millions haven't been able to (lose 100lbs) and plan the next phase of your weight loss journey.
Give it a year for your skin to tighten up. During the next year , work on building up some muscle and slowly burning off the rest of your fat until you get down to about 10-12% bodyfat. This is pretty lean and you shouldn't be able to pinch more skin on your abs between your fingers than you would on your forearm. Your skin should shrink dramatically during this time.
If it still sags a lot after a year and more fat loss, then look into getting a tummy tuck if you want to go down that road. Most good ethical surgeons will require you to lose as much fat as possible anyway before they do this. This is because there is still fat under the skin making it worse and a lot more traumatic on your system to cut all that out.
The only people I've seen with saggy and thin skin after losing all the fat were men who lost over 150-200 lbs or women who lost over 100-150lbs very fast AND also had extremely bad stretch marks. The kind of stretch marks that are as wide as your finger and are as thin as paper. They also had a Lot of them... enough to drastically expand the skin, not just a couple here and there. Your 100 lb weight loss in 8 months wasn't too extreme and I'd put my money on it tightening up and looking good within 6 months to a year as you lean out and add some muscle.
Drink a gallon or more of water per day and eat some "good" fat on most days... Olive oil, fish.
